1. Field of the Invention
This invention relates to a substrate for use in forming an image sensor package and a method of making the substrate.
2. Description of the Related Art
In a typical imaging application, in order to protect the image sensing device in the form of an integrated circuit die, i.e., the image sensor chip, from physical damage and from contaminants in the surrounding environment, it is the practice to place the image sensor chip in a package which includes an opening sealed by a transparent lid thereby allowing the image sensor chip to be exposed to the object to be sensed on an optical principle.
In conventional packaging of these devices, the image sensor chip is mounted on a cofired ceramic substrate with protruding pins by means of an adhesive layer. Typically, the adhesive layer must be cured to fixedly attach the image sensor chip to the ceramic substrate. After wire-bonding and window sealing, the pins are cut to proper length and formed into proper configuration to complete the packaging process.
The ceramic package has many advantages such as low moisture permeability, high dimensional stability, a coefficient of thermal expansion (CTE) closely matching silicon and good thermal conductivity. However, the major disadvantage of the ceramic package is the cost. Therefore, the current trend in the packaging industry is shifting from the ceramic substrate-based packages to organic substrate-based packages because the organic substrate-based packages are less expensive to process and fabricate.
Typically, the image sensor chip is formed of microcrystalline silicon with a coefficient of thermal expansion (CTE) of about 33xc3x9710xe2x88x927/xc2x0 C. However, the organic substrate is usually formed of polymer impregnated fiberglass having a coefficient of thermal expansion of 250xc3x9710xe2x88x927/xc2x0 C. xe2x88x92400xc3x9710xe2x88x927/xc2x0 C. Since there is a significant difference between the image sensor chip and the organic substrate in CTE, the image sensor chip and the organic substrate expand and contract in different amounts along with temperature fluctuations during the curing process of the adhesive layer. This causes warpage of the image sensor chip and the organic substrate thereby resulting in adverse influences that makes the chip unable to detect image. The higher curing temperature and longer curing time are employed, the greater warpage the image sensor chip and the organic substrate will produce. The warped organic substrate and chip will result in adverse influences on the chip itself and the subsequent manufacturing process. The thermal stress due to CTE mismatch may result in delamination between the chip and the substrate or chip cracking. Also, such differences in CTE could induce undesirable strains in the mechanical and electrical connections between the chip and the substrate
The present invention therefore seeks to provide a substrate for packaging the image sensor chip which overcomes, or at least reduces the above-mentioned problems of the prior art.
It is an object of the present invention to provide a substrate for packaging an image sensor chip which mainly comprises a flat chip-supporting member embedded in a main body of thermoplastic material wherein the chip-supporting member is formed from materials with a CTE matching, and the substrate is characterized in that the bottom of the main body is provided with at least two cavities formed corresponding to the comers of the chip-supporting member.
It is another object of the present invention to provide a method of making a substrate for packaging an image sensor chip which utilizes a molding die capable of immobilizing the chip-support member to eliminate the effect of mold flow during molding thereby assuring the coplanarity of the chip-support member.
The substrate of the present invention mainly comprises a main body of thermoplastic material, a flat chip-supporting member embedded in the main body and a lead frame molded integrally with the main body. The lead frame includes a plurality of conductive leads having inner lead portions adapted for coupling to an image sensor chip and outer lead portions for making external electrical connection. The main body has a wall erected from the periphery thereof with a height taller than the height of the image sensor chip. The substrate is characterized in that the bottom of the main body is provided with at least two cavities formed corresponding to the comers of the chip-supporting member.
The present invention further provides a method of making the substrate in accordance with the present invention. The method comprises the steps of: (a) providing a molding die including a top mold and a bottom mold wherein the top mold and the bottom mold define a molding cavity shaped generally to conform to the to-be-molded shape of the substrate, the molding die being characterized in that the bottom mold is provided with at least two projections each having a recess; (b) providing a flat lead frame and a chip-supporting member having a flat chip-supporting surface adapted for receiving an image sensor chip; (c) closing and clamping the molding die on the lead frame in a manner that the chip-supporting member is positioned in the molding cavity wherein the chip-supporting member has portions respectively engaged with the recess of each projection thereby assuring that the chip-supporting surface thereof is parallel to the lead frame; (d) transferring a thermoplastic material into the molding cavity; (e) hardening the thermoplastic material; and (f) unclamping and opening the molding die to take out the molded product.
In the substrate manufacturing method of the present invention, since the comers of the chip-supporting member are engaged and immobilized by the projections of the bottom mold, transferring of thermoplastic material during step (d) will substantially have no effect on the chip-support member thereby assuring that the chip-supporting surface of the chip-supporting member is parallel to the lead frame.